(1) Field of the Invention
The present invention relates to a mobile communication system, a compressed-mode controlling method for the same system, and base and mobile stations in the same system. The invention relates particularly to the art suitable for use in controlling a compressed mode defined by 3GPP (3rd Generation Partnership Project) when different frequency hand over is performed.
(2) Description of Related Art
FIG. 13 is a block diagram showing an example of a W-CDMA (Wideband-Code Division Multiple Access) mobile communication system. The system of FIG. 13 includes, for example, two base stations (BTS) 100 and 200 and one mobile station (MS) 300. The mobile station 300 accesses, via a radio channel, the base stations 100 and 200 in cells 101 and 102, which are formed by the base stations 100 and 200, respectively.
Here, when the mobile station 300, which is in communication with the base station 100, moves from the cell 101 to the adjacent cell 102, the so-called hand over is performed. That is, if the mobile station 300 moves into an overlap area between the cell 101 and the cell 102, or the area where not only a signal from the base station 100 but also a signal from the base station 200 can be received, the mobile station 300 receives a signal from the base station 200, thereby detecting the presence of the base station 200, and switches the target of communication from the base station 100 to the base station 200 at a certain timing (for example, when the signal reception level from the base station 200 exceeds the signal reception level from the base station 100).
At this time, if the frequency used in the cell 101 differs from the frequency used in the cell 102, 3GPP defines that communication processing called a “compressed mode” is to be performed at the time of “hand over”. That is, during the communication performed between the base station 100 and the mobile station 300, the base station 100 notifies the mobile station 300 of the activation of the “compressed mode”, and then compresses at least a part of the transmit data (slot) to the mobile station 300, to make a gap (a transmission gap period in which data transmission is not performed) in the transmission slot, and the mobile station 300 measures the signal reception level of a different frequency from the base station 200 during a transmission gap period. As a result, when the signal reception level from the base station 200 exceeds the signal reception level from the base station 100, the mobile station 300 carries out hand over, switching to the frequency used in the cell 102. Here, in such a “compressed mode”, as the frequency is switched, not only the base station 200 (100) but also the mobile station 300 can compress a part of the uplink slot to the base station 200 (100), to make a gap (transmission gap period) therein. The details of the “compressed mode” are described in the following non-patent document 1.
[Non-Patent Document 1] 3rd Generation Partnership Project (3GPP); Technical Specification Group Radio Access Network; “TS 25.212 V6.4.0 (2005-03) Multiplexing and channel coding (FDD) (Release 6)”.
As already described, when the mobile station 300 moves into the adjacent cell 102, control needs to be performed so that a “compressed mode” is activated when the mobile station 300 comes apart from the base station 100 (cell 101), coming closer to the base station 200 (cell 102). In contrast, when the mobile station 300 is located close to the base station [for example, when the mobile station 300 is close to the base station 100 (in a base station vicinity area 111 in FIG. 14)], which the mobile station 300 is currently in communication with, it is not necessary to activate a “compressed mode”. However, in currently used systems, a “compressed mode” can be erroneously activated even in the above circumstance, so that useless processing is performed by the base station 100, the base station 200, and the mobile station 300.